A St. Louis federal judge has denied class action status for a lawsuit alleging US biotech giant Monsanto and three of its rivals fixed prices of genetically modified seeds.

Judge Rodney W Sippel determined that the plaintiffs would have difficulty proving widespread price-fixing, partly because prices of GM soybean and corn seeds vary widely, reported the Wall Street Journal.

"The dynamics of this localized industry make it highly unlikely that the existence and workings of the alleged conspiracy could be shown through common proof," the judge was quoted by the paper as saying.

Monsanto, Bayer CropScience, Syngenta and DuPont's Pioneer Hi-Bred have all denied claims they plotted to fix genetically modified corn and soybean seed prices.